This invention relates to an electromagnetic valve having an exciting coil to be energized by an a.c. power supply and usable for corrosive fluids.
Electromagnetic valves include an exciting coil which is energized by a d.c. power supply or by an a.c. power supply. D.c. electromagnetic valves are advantageous in that they do not involve chattering but a d.c. power supply must be prepared for the operation of the valve. A.c. electromagnetic valves are operable directly by the commercial a.c. power supply and are therefore widely used. However, the exciting coil of the valve, when energized with a.c. current, produces a magnetic force of varying magnitudes and accordingly a varying attracting force. Consequently the valve stem and other members constituting the valve vibrate at twice the line frequency. This phenomenon is called chattering. Needless to say, chattering impairs the function of the valve.
To prevent chattering, the core of the exciting coil is provided with a shading coil, which usually is a ring of thick copper wire. The change of the magnetic flux of the exciting coil induces an electromotive force on the shading coil, thereby generating a flow of current through the shading coil. The current produces a magnetic flux. Since the flux is produced by the shading coil some time later than the magnetic flux of the exciting coil, the changes of the combined fluxes are smaller than otherwise, whereby chattering is precluded.
On the other hand, when such an electromagnetic valve is used for controlling an aqueous alkali solution or some other corrosive fluid, the fluid will flow to the location of the core of the exciting coil or of the shading coil on the core, possibly causing corrosion to the core or the shading coil. The core which must of course be a magnetic body also needs to have corrosion resistance when the valve is used for corrosive fluids. While magnetic stainless steel is sometimes used for the core, the stainless steel is less resistant to corrosion than nonmagnetic stainless steel. Further the shading coil, which is generally made of copper, is more susceptible to corrosion.